Mourinho appeared on course to record a first victory as United boss at his old club thanks to Anthony Martial, the Frenchman’s brace overturning a half-time deficit.
Martial volleyed home to cancel out Antonio Rudiger’s 21st-minute header before curling in what looked like being the winner from the edge of the penalty area.
However, substitute Barkley – a major injury doubt for the fixture – made sure Maurizio Sarri avoided his first league defeat as Chelsea boss by smashing home a rebound in the sixth and final minute of additional time.
The dramatic finish sparked wild scenes of celebration on and off the field, with Mourinho taking exception to one member of Sarri's staff celebrating right in front of him. Security staff had to get involved to restore order, though the two managers shook hands after an eventful conclusion.
An early kick-off in late October sunshine, combined with a return to domestic duties after an international break, perhaps unsurprisingly led to a slow start before the game's opening goal in the 21st minute.
Rudiger gave Paul Pogba the slip to meet Willian's outswinging corner from the right, planting the ball low beyond visiting goalkeeper David de Gea for his first goal of 2018.
However, Mourinho’s half-time words seemingly transformed United. With the defensive shackles removed after barely venturing out of their own half before the break, they struck twice in an 18-minute spell to take the lead.
Martial capitalised on a slice of good fortune – Ashley Young’s shot taking two deflections to break into his path – to fire in an equaliser, while a heavy touch inadvertently set him up to bend an effort beyond Cesar Azpilicueta's attempted block and put Mourinho on course for a famous triumph.
Yet the Portuguese was denied on his old stomping ground right at the death. David Luiz headed Pedro’s cross against the post and Rudiger hit the rebound straight at De Gea, though it turned out to be third time lucky for the Blues when Barkley hammered his attempt into the net.
What it means: United they stand – until the closing seconds
A slow-burner caught fire in the closing stages, and the antics after Chelsea's second goal will dominate the post-match reaction.
What might annoy Mourinho more, however, is his team’s failure to cling on. In a Jekyll and Hyde display, they remained in their shells for the first 45 minutes before springing into action after the interval. It felt similar to last season's Manchester derby at the Etihad Stadium but, unlike in the 3-2 win over City, they were unable to stand firm through to the final whistle.
Martial at the double
Martial scored twice in a game for United for the first time since November 2016. Both goals offered examples of his finishing abilities when he ends up in the right positions, but flashes of his talent are no longer enough. This should be a springboard for more goals, not the outlier on his season.
Mixed afternoon for Pogba
Pogba epitomised his team’s fortunes. His over-the-top reaction after Chelsea’s goal – he jumped up and down on the spot while complaining team-mate Victor Lindelof had impeded his path to pick up Rudiger – was his only notable moment in the first 45 minutes, yet he was at the forefront of United’s surge to seize control following half-time.
Key Opta stats
95:27 - Ross Barkley's goal was the latest Man Utd have conceded in the Premier League since October 2009, when David Ngog also scored against them at 95:27 for Liverpool. Drama. #CHEMUN pic.twitter.com/pxrpCAuvVz— OptaJoe (@OptaJoe) October 20, 2018
- Chelsea are unbeaten in their last 65 Premier League games at Stamford Bridge in which they’ve been ahead at half-time (W54 D11), last losing against Arsenal in October 2011 (5-3).
- Since his debut in September 2015, Anthony Martial has scored more goals in all competitions for Man Utd than any other player (40 goals).
- Manchester United have kept just one clean sheet from their opening nine Premier League games this season, this after recording seven shutouts at this stage of last season.
- All three of Chelsea’s Antonio Rudiger’s Premier League goals have come in games at Stamford Bridge.
Manchester United welcome back a familiar face when they host Cristiano Ronaldo and Juventus on Tuesday in a tasty Champions League clash, followed by another home game - against Everton - at Old Trafford next weekend. Chelsea, meanwhile, host BATE in Europa League action on Thursday ahead of a Sunday trip to Burnley.